This project is the result of a long-term collaboration between the principal investigator, a basic scientist, and physician scientists and is designed to identify determinants of treatment response in childhood acute lymphoblastic leukemia (ALL). The focus is on methotrexate (MTX) and 6-mercaptopurine (6-MP), antimetabolites that continue to be cornerstones of modern treatment protocols for ALL, and on critical unanswered questions relating to the effective clinical use of these agents. While major progress has been made in treating childhood ALL, this, in part, reflects subgroups comprising nearly 50% of patients with uniquely good prognoses. However, success in the treatment of other patients has lagged behind. We previously demonstrated: (a) a extraordinarily wide range (88-fold) of expression of the human reduced folate carrier (hRFC), the membrane transport system for MTX and tetrahydrofolates, in ALL lymphoblasts, and often disproportionate to levels of MTX transport (b) elevated hRFC expression in good prognosis ALL phenotypes, including hyperdiploidy and t(12;21); and (e) altered hRFC mRNA splice forms, gene sequence variants in the hRFC coding sequence, and a high frequency sequence deletion in a hRFC promoter. Studies were initiated to explore the contributions of (d) the high frequency C677T single nucleotide polymorphism in the methylene tetrahydrofolate reductase (MTHFR) gene and (e) the family of multidrug resistance proteins (MRPs) to chemotherapy response and resistance in pediatric ALLs. In this application, we propose to use primary ALL specimens and ALL xenografts engrafted in NOD-SCID mice to systematically establish: (i) the relationships between patterns of hRFC expression and MTX transport and sensitivities; (ii) the molecular mechanisms that result in the wide range of hRFC expression in primary specimens from the major hRFC-B promoter by in vitro and in vivo assays; (iii) the roles of gene and transcript sequence variants for hRFC and MTHFR as determinants of MTX response and resistance by high-throughput screening of ALL and non-ALL specimens and functional analysis in transfected cells; and (iv) the roles of the MRPs as determinants of MTX and 6-MP accumulations and sensitivities in ALL and transfected cells. Our results should foster the rational chemotherapy of childhood ALL by identifying patients most likely to respond to antimetabolite treatments, and/or who are likely to experience excessive toxicities. Our studies should also lead to new ALL therapies and be relevant to other malignant and non-malignant diseases treated with antifolate drugs. From the key role of hRFC in folate accumulations in mammalian cells and the potential contributions of folate deficiencies to chromosomal instability and malignant transformation, cardiovascular disease, and fetal abnormalities, our results should have far-reaching significance to human heath and disease.